Combine nutella, chocolate and sweetened condensed milk in a microwave safe bowl and stir. Microwave for 20-30 seconds at a time, stirring between each session, until the mixture is smooth and glossy.
While it's being warmed up, place parchment paper in a small rectangular baking dish.
Pour the hot fudge mixture into the baking dish and smooth it out. Sprinkle the flaky salt on top.
Place in the refrigerator for at least 2 hours or until the fudge has hardened.